Learning the ropes

Aug 6, 2010

Ralph Naranjo

Modern sailors and power cruisers may have little recollection of natural fiber lines, but the rope manufacturing process that began centuries ago continues today. The process has been bolstered by better machinery and cutting edge chemistry, allowing miracle fibers to be spun into yarns, twisted into strands and combined into ropes.

Most contemporary cruisers started messing around in small boats well after manila, sisal, hemp and jute were allocated to forgotten corners of old chandleries or put on display at the Mystic Seaport Museum. The aroma of tarred marlin and reels of manila anchor rode linger in the memory of “old salts,” but today the term rope has become synonymous with nylon and Dacron (polyester) cordage.

These miracle polymers are grown in the lab and deliver better mechanical properties than natural fibers, plus they eliminate the age old problem of rot. They begin life as filaments thinner than a human hair, and can be bundled in much the same way grown fibers were handled, but the resulting line emerges as a stronger, more supple and tougher cordage. Nylon and Dacron were anything but the end of the road for the chemists and engineers who continue the hunt for lighter weight materials with better mechanical properties.

The old industrial slogan, “better living through chemistry” underscores the theme of the rope revolution. Dacron and nylon may have been the prototypes, but today all the buzz is about Dyneema, Vectran, PBO and other unpronounceable fiber names that are getting stranded, braided, plaited and tucked inside familiar polyester or polypropylene line covers. Skeptics assume it’s marketing spin, a fancy line to tie up the emperor’s new pants. But look at the data with an engineer’s curiosity and the numbers speak for themselves.

Better numbers mean more strength
The numbers highlight the difference among similar diameter lines, and by looking at specs such as the tensile strength of new cordage, one sees a clear difference between look-alike lines on similar size spools. Pull-to-destruction test results are accrued by tensioning test samples set up on a sturdy metal jig and hauling away with a hydraulic ram, screw jack or a powerful winch. As the load increases, the rope sample stretches, eventually reaching its yield point where elastic deformation becomes plastic. This is the point where there is no longer any spring back if the load ceases. If tension continues to increase, catastrophic filament failure occurs and the line breaks. A line with high tensile strength and minimal elasticity will usually offer very little yield, and massive filament rupture will occur rather than any prolonged period of plastic deformation.

Shock loading is both difficult to quantify and hard to predict. It results from short-lived spikes in the tensile load, and cordage with minimal ability to elongate tends to suffer greater damage from such momentary increases in tension. Nylon’s elasticity can act as an energy absorber dampening short-lived shock loads. This ability to stretch may be a good characteristic in an anchor rode or a spring line, but it’s far from advantageous in a halyard. When it comes to low stretch cordage, shock loads are handled by designing an appropriate safety factor into the rigging system. This is why manufacturers provide safe working load data for specific uses and these numbers may be three or more times smaller than the actual breaking strength of the cordage. The Cordage Institute, the rope industry’s technical body, specifies a safety factor of 5-12 for non-critical use, and a safety factor of 15 for lifelines! Keeping the loading at such a small percentage of the breaking strength of the line also minimizes stretch and prolongs the lifespan of the cordage.

Creep is another interesting mechanical factor, and lines that remain under constant load are most affected. One research team in the UK found that loading polyester rope to 50 percent of its breaking strength equated with a 100-year lifespan of the material in that use. This was a dead weight study, no inertial loads, UV, abrasion or other wear-and-tear factors were involved. Using the same scenario, the load was increased to 64 percent of the breaking load and the lifespan of the line in this configuration was shortened to one year. Load and lifespan are obviously inversely proportioned and nonlinear in their relationship.

Creep has several stages, and in its primary phase there’s a relatively abrupt elongation as filaments share loading and the physical dynamics within the rope conform to the stress. The secondary phase in the creep phenomena is more linear and slower to occur, but in the tertiary phase, filament snapping can actually be heard and elongation becomes more rapid, continuing to accelerate up to the point of complete failure. The bottom line when it comes to creep is to choose a cordage type and diameter that keeps the working load well south of 50 percent of the breaking strength under the most adverse circumstances, and under 25 percent if true longevity is of interest. Creep tends to be more common in Olefin-based lines and almost nil with high-end fibers such as Zylon-based PBO.

In the lab, researchers continue the quest for polymers that form repeatable long chain links, and in some cases even form crystalline lattices with immense tensile strength. Carbon fiber is a classic example of such a eureka moment. Researchers discovered that by heating inexpensive poly-acrylic-nitrile fiber to about 2,000° C in the presence of pitch, the common inexpensive T-shirt fiber turns into crystalline carbon — a filament with an exponential increase in tensile strength. Such lead-to-gold discoveries represent the forward strides made in chemistry, advances that have been leveraged by the cordage industry. But regardless of the recipe, good cordage, like good pancakes, relies just as much on how the batter is handled.

When it comes to rope manufacture, the raw material is the fiber bought in bulk by each cordage company, and picking the right filament formula and fiber blend is a big deal. The process mimics the old software adage “garbage in, garbage out.” Quality cordage requires top notch raw materials, plus quality control in how yarns are twisted, braided or paralleled. Each manufacturer spends an inordinate amount of time making sure that the spools of raw material that they invest in are both high in quality and cost effective. Every manufacturer knows what batch inconsistency or supply disruptions can do to their brand and top label companies like New England Ropes, Samson and Yale work as hard to maintain product consistency as they do to introduce the next new miracle fiber.

The bottom line
By comparing the tensile strength of a variety of 7/16-inch lines, a crew will get a feel for how wide a range of rope options are available. Independent testing has shown that three-strand nylon rates a 5,900-pound breaking strength, while a high-quality Dacron (polyester) braid such as Sta-Set tallies up a 6,600-pound tensile strength rating. New England Ropes’ Endura 12 and their Pro-PBO, Samson’s Progen II and Yale’s PoBOn deliver an astounding tensile load rating of 24,000 pounds. These lines are the same diameter as the Dacron and nylon mentioned above, but they are four times stronger!

Elongation, or stretch, is another big deal when it comes to line choice. Here we are talking about the elastic phase of stretch that occurs with repetitive loading. It’s in this realm that high-tech fibers really earn their keep. Take for example similar diameter lines that are each 100 feet long and loaded to 30 percent of their braking strength. The higher end PBO, Vectran and Dyneema (Spectra) products will stretch less than a foot under such load while conventional lower-priced polyester products will add three feet to their length. Combine this data with the fact that 30 percent of the breaking strength of a top-end line is about triple the strength of a lower-cost Dacron product, and you can see that stretch minimization per equal load is even more impressive. The bottom line is that stronger, less stretchy, lighter lines are like better quality tools — likely to be worth the investment in the long run.

Tensile strength and elongation are by no means the only consideration when it comes to cordage consideration. Creep (elongation under load), UV stability, abrasion resistance handleability and cost are also key factors. For example, when it comes to replacing a wire halyard with rope, stretch is only one factor. Other variables that need to be added to the mix are issues such as the manufacturer’s line diameter to sheave radius ratio, or how hard the cordage can bend without harming the fiber bundles. There’s also some concern about the cover material and special coatings on the line that help keep UV and chemical damage, caused by atmospheric deposition (acid rain, etc.), at a minimum. In short, it’s important to consider the full range of factors influencing line choice when selecting the right cordage for a specific application. This includes cost, and the per foot price of 7/16-inch diameter cordage in many chandleries ranges from just over one dollar per foot to just under five dollars per foot. In the mid point of this price spread is where high-quality double braid polyester begins to give way to partial Dyneema cores, and the result is a significant upturn in performance.

Sheets are another classic example of balanced decision making. Limiting stretch is a good thing, but other factors must also be taken into consideration. Just ask anyone in the crew who’s handling a genoa sheet while tacking up a tight channel, and their preference will lean toward the line with the best handling characteristics. This includes its suppleness, ease with which it wraps around a winch, and how it grips and releases the surface of the drum. Other important factors are how the line holds and releases from the jaws of a self tailing winch and how the cover holds up under the repeated clamping of a rope clutch. There’s even an interest in how easily the line coils and its willingness to remain hockle free.

Putting special purpose cordage to use
The racing sailor doesn’t need much convincing to see the value in less stretch, lighter weight and increased strength. Voyagers often take a little more convincing in order to let o of their nylon for anchor rode and dock lines and polyester for all running rigging tradition. However, a closer look at what’s at stake can turn the tide. Take for example, the value of a Dyneema (Spectra)-cored halyard with a tough, UV protective polyester cover and only one-third the stretch of a Dacron sequel. One of the most important upsides of such a halyard is how it behaves in a gust. Its reluctance to stretch keeps a sail’s luff from loosening and prevents increased draft when it’s least desirable. A taught luff in heavy weather is essential and with many cruisers preferring to run halyards all the way back to the cockpit, even a small stretch percentage can add up to significant luff sag.

Another very beneficial use of high modulus cordage is as a replacement for wire running backstays. Voyagers making long ocean passages benefit from the extra support and column control afforded by runners, not to mention their mandatory use with a heavy weather forestaysail. But in light air it’s nice to be able to tie them off near the shrouds and sail sans runners. Using an all-line option, that eliminates the need for a wire stay and rope tail combination, makes handling a runner tricky.

Those who are serious about minimizing diesel fuel usage while cruising under sail tend to gravitate toward light air reaching sails and asymmetric spinnakers. No foil, endless line furlers and spinnaker socks also benefit from light, easy to handle cordage, but the biggest use in this realm is the value of light air sheets such as New England Ropes’ Flight Line, a polypropylene braided cover over a Dyneema core. It’s strong, non-stretchy and very light — a big plus in ghosting conditions.

There are even specialized braided dinghy painters that are abrasion resistant, UV stable and comprised of a core that’s buoyant. Keeping the line on the surface makes wrapping the painter in the prop a difficult maneuver to execute.

But the big news in painters revolves around the way major brand rope manufacturers are responding to the age old challenge of mooring a vessel and providing the best possible chafe protection. Post-storm rope forensics has shown that friction from stretching nylon filaments can cause enough heat to melt these tiny fibers.

Today there’s a move toward a blended painter technology with a tad less stretch. Yale’s Maxi-Moor is comprised of a Polydyne line (polyester braided cover with a nylon core) over which a urethane jacket has been added. Splicing is done at the factory and a piece of built-in chafe gear can be slid into position where it’s most needed.

When all is said and done, nylon and Dacron line does more than an adequate job aboard a voyaging boat, and it remains the price point favorite, but special purpose higher modulus cordage is the top grade solution for those looking for less stretchy halyards and runners, lighter drifter reacher sheets and more secure mooring pennants — all worthy of a voyager’s interest and investment.

Ralph Naranjo is a circumnavigator and a marine technical writer based in Annapolis, Md. He is the author of the books Wind Shadow West and Boatyards and Marinas.